mercruiser 5.7 sudden milky oil

hey ! i,m asking for a friends boat where we try to figure out what happened.

he bought a used mercruiser 5.7 and we installed it on his boat. the engine started and ran fine without issues ( at the dock) .

now we tried a first ride. first the engine was warmed up at the dock and everything was fine- then we were able to make 90 . but not mph , feet...

after leaving the dock virtually after first aplly of throttle ( i,m not talking gunning the engine, just a smooth aplly to roughly 2000 rpm) the engine started to struggle, misfire and finally died.

after checking the little things i also pulled the dipstick ( i did it also before the ride and it was ok !!) and saw all the milky oil with an oil level easily 2-3 quarts above maximum.

we pulled all plugs and in none of the 8 cylinders there is any water.

my question : beyond the obvious thing of a cracked block ,

- can a head gasket fail in a way that water passage opens directly to the oil and you in short time get massive water to the oil without having water in cylinders ?

-the exhaust risers i think can be excluded- i do not see a way how bad rises would NOT put water into cyliders but directly to the oil or am i wrong?

-does the intake manifold have water passages on this V8 ?

- any other ideas what might happened in such a sudden way?

when it comes to a cracked block.- the engine was running at the dock. not few minutes, lets say a good hour. the oil level was checked several times, everything was ok , no cracks were found when buying it, compression was tested at purchase and in the 140 -150 psi region.

thanks much in advance !

Re: mercruiser 5.7 sudden milky oil

Ayuh,.... My 1st guess is a cracked block,...
though there is the outside chance the intake manifold didn't seal,....

It's an even smaller chance it's the head gasket, but I doubt it...

Re: mercruiser 5.7 sudden milky oil

Pressure check the engine block....that's were I would start ..

Re: mercruiser 5.7 sudden milky oil

thanks for quick reply gents !

Bond-o said:

Ayuh,.... My 1st guess is a cracked block,...
though there is the outside chance the intake manifold didn't seal,....

Click to expand...

is there a way the not sealed intake manifold will flood quickly and massivly the oil sump directly without putting water into cylinders ?

when we have a situation with for sure NO water into cylinders but massive water in the oil sump, can it be at least theoretically anything else than a cracked block at all ?

pressure testing the block may tend to be difficult, we do not have the tools and he does not even has a trailer for the boat. i could borrow him mine to get it to a mercruiser dealer- maybe.

i also believe the block is gone, but of course i hope for him its something minor. i am confused because i saw the engine with my own eyes idling for an hour , not at muffs, in water. i personally checked the oil level after this and it was fine ! it must happened suddenly when we tried our first ride.

so what ? a small crack that finally opened after advancing power a little ?

Re: mercruiser 5.7 sudden milky oil

Ayuh,.... Water flow, 'n pressure is higher at 2000 rpms, than at idle....

Pull the intake, 'n Look at the water ports for a leak... 'n the casting bottom for cracks...

Re: mercruiser 5.7 sudden milky oil

You mentioned buying a used mercruiser 5.7 . . . and that it was the first time out. Perhaps the engine was for sale on the used market because it had this problem . . . therefore it probably did not come on suddenly, but was there all the time.

As mentioned, the best thing to do would be a pressure test of the cooling system. You could also do the more typical cylinder compression test and leak tests just to see the overall condition of the engine. All of these tests should help in understanding the problem before you take the engine apart, etc.

Re: mercruiser 5.7 sudden milky oil

Bond-o said:

Ayuh,.... Water flow, 'n pressure is higher at 2000 rpms, than at idle....

Pull the intake, 'n Look at the water ports for a leak... 'n the casting bottom for cracks...

Click to expand...

thanks bond, will try this . but- a cracked intake manifold would let water via the intake valve to the cylinders which then finally may alo get to the sump via the piston ring or am i wrong ?

again - no a little bit of water in the cylinders !

and two or more litres in maybe 2 minutes of ride- would it not hydrolock in this case ?

@ tpenfield : it can of course be an internally shot engine from the previous owner. we can only say that any freezing damage the last weeks in germany can be excluded, but we cannot say what happened to the engine before. overall condition and compression was tested before purchase. and correct- it was the first time out in our hands with the described result.

Re: mercruiser 5.7 sudden milky oil

If you have water in the crankcase but not the cylinders then you could have a leak between a water jacket or gasket seal DIRECTLY into the crankcase.

And yes anything that houses water is a possible contributor.


That said-- If you can do the coooling system pressure test before you take things apart you will most likely be glad you did.


My guess--- I will be surprised if you do not have a cracked block or passage that was damaged in a freeze before you purchased the motor. But it could be something simpler & lets hope it is.


Good luck-- These things can be upsetting.

Re: mercruiser 5.7 sudden milky oil

aerobat said:

thanks bond, will try this . but- a cracked intake manifold would let water via the intake valve to the cylinders which then finally may alo get to the sump via the piston ring or am i wrong ?

again - no a little bit of water in the cylinders !

and two or more litres in maybe 2 minutes of ride- would it not hydrolock in this case ?

@ tpenfield : it can of course be an internally shot engine from the previous owner. we can only say that any freezing damage the last weeks in germany can be excluded, but we cannot say what happened to the engine before. overall condition and compression was tested before purchase. and correct- it was the first time out in our hands with the described result.

Click to expand...

Ayuh,... The Intake manifold has a top, an inside,'n a Bottom, which is common to the lifter valley, which drains to the base...

Pull the Intake,...

I'm thinkin' you'll see cracks on it's bottom, or worst yet, in the lifter valley of the block...